Many wireless local area networks (WLANs), such as those based on a communication protocol that is compatible with an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard (which is sometimes referred to as ‘Wi-Fi’), involve contention-based distributed access systems. In particular, the WLANs are usually contention based because they typically utilize unlicensed frequency bands or spectra, which are unpredictable and are often subject to interference.
For example, Wi-Fi access is often based on carrier sense multiple access (CSMA) with a collision avoidance technique, such as single-user transmission via enhanced distributed channel access (EDCA). EDCA usually involves random access to the shared communication channel or medium by contending electronic devices (which are sometimes referred to as ‘stations’ or STAs). In particular, as shown in FIG. 1, which presents an existing collision avoidance technique, air access parameters (the arbitration interframe spacing number or AIFSN, the minimum and maximum contention window or CW, etc.) are usually specified by an access point in a WLAN. Moreover, an electronic device in the WLAN is usually only allowed to transmit once a physical channel clear assessment (CCA) and a virtual CCA or carrier-sensing technique (such as the network allocation vector or NAV) are both clear. Typically, the electronic device selects a random back-off value N between 0 and the CW. Furthermore, the back-off time in the electronic device usually expires only when the communication medium is idle for a time duration greater than the AIFS plus N slots. However, if the back-off timers in two electronic devices reach a back-off value of zero concurrently, a transmit collision occurs, and the CW is typically doubled. Therefore, in the event of a transmit collision, both of the electronic devices can be penalized.
Consequently, the reliability of EDCA typically depends on the number of electronic devices contending for the communication medium. In particular, when there are four or more electronic devices concurrently contending to transmit on the communication channels, EDCA usually results in a high rate of collisions (e.g., above 30%). Therefore, Wi-Fi based on EDCA may not suitable for use with medium-bandwidth or high-bandwidth applications (e.g., video streaming, mirroring, gaming, etc.) in dense communication channels.
The unpredictability of the interference with EDCA can make coordination across multiple electronic devices challenging (especially for an unmanaged WLAN), and, as described above, can result in the failure of a collision free period (CFP). In order to address these challenges, contention-free multi-user transmission in uplink has recently been proposed in the IEEE 802.11ax standard. This approach can dramatically change how an electronic device accesses the communication medium. In particular, an electronic device can transmit without contending for the communication medium. Instead, an access point may contend for the communication medium on behalf of the electronic device, and may grant transmission opportunities to the electronic device using a trigger frame (which is sometimes referred to as ‘trigger-based access’ or ‘trigger-based channel access,’ e.g., uplink multi-user transmission). During trigger-based uplink channel access, an access point may sense the communication medium and, as needed, perform a back-off procedure on behalf of potential uplink trigger-access-enabled electronic devices. Then, the access point may send a trigger frame with multi-user allocation information to the electronic devices. In response to the trigger frame, the electronic devices may send uplink traffic in the allocated-resource units in a synchronized manner in a multi-user transmission.
In principle, the use of trigger-based access and multi-user transmission can significantly reduce the contention by the electronic devices in the WLAN. Consequently, trigger-based access is often expected to result in improved communication performance.
However, trigger-based access and multi-user transmission is not backwards compatible with existing electronic devices, and may not be suitable for use with point-to-point ad-hoc links, such as Apple Direct Wireless Link or AWDL (from Apple Inc. of Cupertino, Calif.), Neighborhood Area Network (NAN), and/or Wi-Fi Direct.
Alternative approaches to trigger-based access include the use of a dynamic channel selection technique to select a ‘quiet’ communication channel to avoid collisions. However, the dynamic channel selection technique is typically not an option when there are too few communication channels, such as in the 2.4 GHz frequency band. Moreover, this approach typically adds channel synchronization overhead, which usually consumes bandwidth and increases latency. Furthermore, the dynamic channel selection technique is also not backwards compatible with existing or legacy electronic devices.